Purchasing them now or later closer to your vacation is strickly up to you and will have no effect upon whether or not the ticket will be accepted at the entrance. If you purchase through a reputable ticket broker or WDW you will can rest assured that your ticket will work. In fact, when you check in at your hotel, have the paper tickets transferred to your hotel key...in that way you only have one ticket/key to keep track of and you will know right then and there if the ticket is valid.
THe only advantage of purchasing now is to avoid a possible price increase between now and your trip.